I dont think there is a fixed length other than as BS says that the first 2 digits identify the region, then the remaining letters id the exchange. For example a short snippit.
ESABB ABBEYHILL FTTC & FOD Now City of Edinburgh
EMABBOT ABBOTS BROMLEY FTTC Now East Staffordshire District (B)
EMABRIP ABBOTS RIPTON FTTC/P & FOD Now Huntingdonshire District
STABTSY ABBOTSBURY FTTC/P Now West Dorset District
SWAAI ABERAERON FTTC/P Now Sir Ceredigion - Ceredigion
NSABC ABERCHIRDER FTTC Now Aberdeenshire
WNABC ABERCONWY FTTC/P Now Conwy - Conwy
SWAAV ABERCRAVE FTTC/P Now Powys - Powys
SWABT ABERCYNON FTTC & FOD Now Rhondda, Cynon, Taf - Rhondda, Cynon, Taff
SWAA ABERDARE FTTC/P Now Rhondda, Cynon, Taf - Rhondda, Cynon, Taff